The present invention relates to a cathode active material comprising two kinds of lithium transition metal complex oxides with different chemical compositions, and a secondary battery using the cathode active material.
In recent years, in accordance with advances in electronic technologies, a large number of small-sized portable electronic devices such as camera/VTR (video tape recorder) combination systems, cellular phones, laptop computers or the like have come into widespread, and a development for reducing their sizes and weights has proceeded. Accordingly, a development of compact and lightweight batteries having a high energy density, more specifically secondary batteries as portable power sources used in the portable electronic devices has proceeded. Among them, a lithium-ion secondary battery holds great promise, because the lithium-ion battery can obtain a higher energy density than a lead acid battery or a nickel-cadmium battery using a conventional liquid electrolyte in which water is used as a solvent.
As a cathode active material of the lithium-ion secondary battery, for example, a lithium-cobalt complex oxide and a lithium-nickel complex oxide both having a layered rock salt structure, and a lithium-manganese complex oxide having a spinel structure are practically used. Each of them has advantages and disadvantages, but at present, the lithium-cobalt complex oxide offers the most excellent balance of various aspects such as capacity, cost and thermostability, and is widely used. On the other hand, the lithium-manganese complex oxide has some disadvantages such as lower capacity, slightly poor high temperature storage characteristics or the like, and the lithium-nickel complex oxide has some disadvantages such as slightly lower thermostability and slightly lower stability of a crystalline structure, poor cycle characteristics, lower environmental resistance or the like, however, they are superior in costs and stable supply of materials, so studies of these oxides have been conducted to make use of these oxides in future.
Specifically, the lithium-nickel complex oxide is a promising material for improving the energy density, because the lithium-nickel complex oxide has a higher charge-discharge capacity. Therefore, a large number of methods of stabilizing the crystalline structure of the lithium-nickel complex oxide such as a method of substituting a different kind of element to form a solid solution, or the like have been proposed (refer to J. Solid. State. Elect rochem. 3 (1999)121 or the like).
However, in the method of substituting a different kind of element, in general, a crystalline lattice can be strengthened, and the cycle characteristics and storage characteristics can be improved, however, the method often involves a decline in the capacity or a decline in electronic conductivity and ionic conductivity. Therefore, there is a disadvantage against discharge of a large current. Moreover, with a trend in recent years toward higher density and higher speed of integrated circuits, or a growing range of uses of high power sources such as electric vehicles or the like as a backdrop, a demand for large current discharge characteristics has been growing.
In addition, in order to prevent a decline in the electronic conductivity, it is effective to increase an amount of a carbon material added to a cathode as an electronic conductor. However, this method is not preferable, because an amount of the cathode active material filled is reduced, thereby resulting in a decline in a battery capacity.
In view of the foregoing, it is an object of the present invention to provide a cathode active material exhibiting a higher capacity and superior cycle characteristics and capable of obtaining a sufficient discharge capacity during large current discharge, and a secondary battery using the cathode active material.